The present invention relates to a method of producing a phenol compound from an aromatic aldehyde. The phenol compound is a very useful compound as a material or intermediate for producing a synthetic resin, an insecticide, an antioxidant, etc.
The phenol compound such as phenol, alkylphenol, etc. has been produced by separation of tar acids or various synthetic methods such as a cymene method. However, a high-purity phenol compound is difficult to obtain by the separation due to small difference in the physical properties between the isomers as well as the homologies. In the synthetic methods, a sulfonation-alkali fusion method is disadvantageous because isomers are by-produced and waste disposal is required. The cymene method requires complicated facilities.
One of other known synthetic method is a method utilizing Baeyer-Villiger reaction, in which the phenol compound is produced through a reaction between the aromatic aldehyde and a peroxide such as peracetic acid, perbenzoic acid, etc. This method is suitable for producing a high-purity phenol compound with less isomers.
Although Baeyer-Villiger reaction generally provides the phenol compound quantitatively in the reaction between an aromatic aldehyde having an electron donative group and a peroxide, provides a large amount of aromatic carboxylic acid with a little phenol compound in the reaction between an aromatic aldehyde having an electron attractive group and an peroxide. Therefore, various proposals have been made to increase the yield of the phenol compound.
For example, Japanese Patent Application Laid-Open No. 47-27933 proposes to quantitatively synthesize the alkylphenol from an aromatic aldehyde using an organic peracid derived from a carboxylic acid showing pKa smaller than 4 in water.
Japanese Patent Application Laid-Open No. 48-56635 proposes a method where the alkylphenol is obtained by a reaction between hydrogen peroxide and an aromatic aldehyde in the presence of an inorganic acid.
However, since performic acid, peracetic acid, hydrogen peroxide, etc. used in these methods are relatively expensive and have danger of explosion, special care has to be made in handling them.
To avoid the danger of direct handling, Japanese Patent Application Laid-Open No. 1-121229 proposes a method of producing a phenol compound, where an aqueous hydrogen peroxide is added to a solution of an aromatic aldehyde in formic acid to form performic acid in situ in the reaction system, and the aromatic aldehyde is oxidized by performic acid to an aryl formate which is then hydrolyzed. However, this method involves the following problems:
(1) Although the method is carried out safely as compared with a method directly utilizing an organic peracid such as performic acid, perbenzoic acid and peracetic acid, a cyclic perether is generated by the reaction between the aromatic aldehyde and hydrogen peroxide. The cyclic perether is in danger of explosion and requires very expensive facilities for safety. PA0 (2) A high-boiling product is produced in the step of forming the aryl formate by the reaction between hydrogen peroxide and a phenol compound formed from a part of the synthesized aryl formate. This reduces the yield of the phenol compound. PA0 (1) The unreacted aromatic aldehyde and its corresponding aryl formate are hardly separated from each other by distillation due to the extremely small difference between the boiling points. For example, the boiling point of 2,4-dimethylbenzaldehyde is 117.degree. C./25 Torr, while 114.degree. C./25 Torr for 2,4-xylyl formate. PA0 (2) An approach avoiding the above problem is to first completely oxidize the aromatic aldehyde to an aromatic peracid thereby reducing the amount of the unreacted aromatic aldehyde. Thereafter, the aromatic peracid is reacted with freshly supplied aromatic aldehyde to produce the aryl formate and the aromatic carboxylic acid. However, this method requires two-stage reaction and increases the selectivity of the aromatic carboxylic acid with significant decrease in the selectivity of the aryl formate.
Further proposed is a method using another oxidizing agent in place of the peroxide. U.S. Pat. No. 5,237,092 proposes a method where an aromatic aldehyde is oxidized in a water-free condition to an aryl formate and an aromatic carboxylic acid using an oxygen-containing gas as the oxidizing agent. Although this method uses an inexpensive oxidizing agent and is easy to practice, it involves the following problems: